Typically iodine-123 is injected or prescribed by pill; the patient is later imaged by a nuclear camera. Areas where the radioactive iodine (called radio-iodine) concentrates will show up on the nuclear camera.
Iodine-123 is most commonly used to detect cancers of the thyroid, as this is the organ most receptive to forms of iodine. Once the thyroid has absorbed the radio-iodine, any cancer present will have a differing uptake of radio-iodine than the natural, surrounding tissue. This difference can be analyzed by a doctor to determine whether there exists a possibility for thyroid cancer. A tissue biopsy may also be performed to further determine whether it is cancerous or not, and whether it is malignant or benign.